Heat exchangers between liquids and gases are characterized by primary surfaces on the liqud side, and in addition secondary surfaces of the gas side. The predominant configuration is the finned heat exchanger tube type. It is possible with these, to increase the surface on that side of the heat-permeable wall where the gas is conveyed up to 10 times the surface on the wet side of said wall. The heat transfer coefficient of gas is 50 to 100 times smaller than the heat transfer coefficient on the wet side. Therefore, the surface ratio obtainable by finned heat exchanger tubes is much smaller than necessary for optimum heat transfer, resulting in large heat exchanger sizes.